What would you do?

So I have a dilemma to run by you ladies for your opinions. I have a 1 year old and he was EBF until about 9.5 months at which point I had to transition to formula due to losing my supply from my new pregnancy. I'm due in April when my son is 1 year 4 months at which point I will BF the newborn and I plan on pumping and giving him my milk in a sippy cup. Dilemma is switching off formula to another milk before then or keep him on formula for 4 months and then back to BM and then switch him after 2+ however long I give him my milk? Just looking for opinions here as to what you would do with your child. TIA!